2.7.1 Dynamic Default Border Configuration
| Item |
Details |
| Organization |
Abhavtech.com |
| Version |
2.0 |
| Last Updated |
December 2025 |
1. Overview
1.1 What is Dynamic Default Border?
Dynamic Default Border is a feature in SD-Access that allows edge nodes to dynamically select the optimal border node for external (default route) traffic, rather than using a statically configured border.
┌─────────────────────────────────────────────────────────────────────┐
│ Static vs Dynamic Border Selection │
├─────────────────────────────────────────────────────────────────────┤
│ │
│ STATIC BORDER (Traditional) DYNAMIC BORDER (Modern) │
│ ┌─────────────────────┐ ┌─────────────────────┐ │
│ │ │ │ │ │
│ │ Edge-1 ──► Border-1│ │ Edge-1 ──┬► Border-1│ │
│ │ Edge-2 ──► Border-1│ │ └► Border-2│ │
│ │ Edge-3 ──► Border-2│ │ │ │
│ │ Edge-4 ──► Border-2│ │ Edge-2 ──┬► Border-1│ │
│ │ │ │ └► Border-2│ │
│ │ (Fixed assignment) │ │ │ │
│ └─────────────────────┘ │ (Dynamic selection │ │
│ │ based on metrics) │ │
│ Problems: └─────────────────────┘ │
│ • Manual configuration │
│ • No automatic failover Benefits: │
│ • Suboptimal paths • Automatic selection │
│ • Border overload possible • Built-in redundancy │
│ • Optimal path selection │
│ • Load distribution │
│ │
└─────────────────────────────────────────────────────────────────────┘
1.2 When to Use Dynamic Default Border
| Scenario |
Recommendation |
| Multiple border nodes at site |
✅ Recommended |
| Single border node |
Not applicable |
| LISP Pub/Sub enabled |
✅ Required for best results |
| SD-Access Transit |
✅ Required |
| Large campus (>500 endpoints) |
✅ Recommended |
| Branch with Fabric-in-a-Box |
Optional |
2. Abhavtech Dynamic Border Design
2.1 Site-Level Configuration
| Site |
Border Nodes |
Dynamic Default |
Reason |
| Mumbai |
MUM-BN-01, MUM-BN-02 |
✅ Enabled |
Dual border redundancy |
| Chennai |
CHE-BN-01, CHE-BN-02 |
✅ Enabled |
Dual border redundancy |
| London |
LON-BN-01, LON-BN-02 |
✅ Enabled |
Dual border redundancy |
| Frankfurt |
FRA-BN-01, FRA-BN-02 |
✅ Enabled |
Dual border redundancy |
| New Jersey |
NJ-BN-01, NJ-BN-02 |
✅ Enabled |
Dual border redundancy |
| Dallas |
DAL-BN-01, DAL-BN-02 |
✅ Enabled |
Dual border redundancy |
| Bangalore |
FiaB (integrated) |
❌ N/A |
Single border (FiaB) |
2.2 Architecture Diagram
┌─────────────────────────────────────────────────────────────────────┐
│ Mumbai Site - Dynamic Default Border │
├─────────────────────────────────────────────────────────────────────┤
│ │
│ ┌─────────────────────┐ │
│ │ External/WAN │ │
│ │ (SD-WAN/MPLS) │ │
│ └──────────┬──────────┘ │
│ │ │
│ ┌────────────────┼────────────────┐ │
│ │ │ │ │
│ ▼ ▼ ▼ │
│ ┌─────────────────┐ ┌─────────────────┐ │
│ │ MUM-BN-01 │ │ MUM-BN-02 │ │
│ │ (Default Border)│ │ (Default Border)│ │
│ │ Priority: 1 │ │ Priority: 2 │ │
│ └────────┬────────┘ └────────┬────────┘ │
│ │ │ │
│ │ LISP Pub/Sub │ │
│ │ ◄──────────────────►│ │
│ │ Default Route │ │
│ │ Advertisement │ │
│ │ │ │
│ ┌────────┴──────────────────────┴────────┐ │
│ │ │ │
│ │ Control Plane Nodes │ │
│ │ (MUM-CP-01, MUM-CP-02) │ │
│ │ │ │
│ └────────────────┬───────────────────────┘ │
│ │ │
│ │ Dynamic Default │
│ │ Route Selection │
│ │ │
│ ┌────────────────┼────────────────────────────────┐ │
│ │ │ │ │
│ ▼ ▼ ▼ ▼ │
│ ┌───────┐ ┌───────┐ ┌───────┐ ┌───────┐ │
│ │MUM-ED │ │MUM-ED │ │MUM-ED │ │MUM-ED │ │
│ │ -01 │ │ -02 │ │ -03 │ │ -04 │ │
│ └───────┘ └───────┘ └───────┘ └───────┘ │
│ │
│ Each edge node independently selects optimal border │
│ based on LISP metrics and path cost │
│ │
└─────────────────────────────────────────────────────────────────────┘
3. Catalyst Center Configuration
3.1 Enable Dynamic Default Border
Catalyst Center → Provision → Fabric Sites → [Site] → Fabric Infrastructure
1. Select Border Node
2. Click "Edit"
3. Under "Border Settings":
☑ Enable as Default Border
☑ Dynamic Default Border (new option with Pub/Sub)
4. Set Border Priority (optional):
Border Priority: 1 (lower = preferred)
5. Click "Save"
6. Repeat for second border node with Priority: 2
3.2 Verify Configuration
Provision → Fabric Sites → [Site] → Fabric Infrastructure
Border Node View:
┌──────────────────────────────────────────────────────────────┐
│ Device │ Role │ Default │ Dynamic │ Priority│
├──────────────────────────────────────────────────────────────┤
│ MUM-BN-01 │ Border │ Yes │ Yes │ 1 │
│ MUM-BN-02 │ Border │ Yes │ Yes │ 2 │
│ MUM-CP-01 │ Control Plane │ - │ - │ - │
│ MUM-CP-02 │ Control Plane │ - │ - │ - │
│ MUM-ED-01 │ Edge │ - │ - │ - │
└──────────────────────────────────────────────────────────────┘
4. CLI Configuration
4.1 Border Node Configuration
! On MUM-BN-01 (Primary Border)
! Dynamic Default Border is automatic with LISP Pub/Sub
! Verify LISP Pub/Sub is enabled
router lisp
locator-set RLOC
IPv4-interface Loopback0 priority 1 weight 100
exit-locator-set
!
service ethernet
encapsulation vxlan
default-etr-locator-set RLOC
! Border advertises default route to control plane
! This happens automatically when:
! 1. Border has external connectivity (fusion/handoff)
! 2. Border is marked as "Default Border" in Catalyst Center
! 3. LISP Pub/Sub is enabled
! Verify default route advertisement
show lisp instance-id * ethernet publication
4.2 Edge Node Behavior
! On MUM-ED-01 (Edge Node)
! Edge nodes receive default route dynamically
! Check default route in fabric VRF
show ip route vrf VN_CORPORATE 0.0.0.0
! Expected output:
! Routing entry for 0.0.0.0/0
! Known via "lisp", distance 210
! Tag 4097, type LISP-dynamic
! Last update from 10.100.255.1 (MUM-BN-01) via Vlan4097
! Last update from 10.100.255.2 (MUM-BN-02) via Vlan4097
! Check LISP default route subscription
show lisp instance-id * ethernet map-cache
5. Border Selection Criteria
5.1 Selection Algorithm
Border_Selection_Criteria:
1_Reachability:
- Border must be reachable via underlay
- IS-IS adjacency must be UP
2_LISP_Registration:
- Border must be registered with Control Plane
- EID prefix must be advertised
3_Priority:
- Lower priority value preferred
- Priority 1 selected before Priority 2
4_Path_Cost:
- IS-IS metric to border node
- Lower cost preferred
5_Load_Distribution:
- With equal priority/cost, load is distributed
- ECMP-like behavior across borders
5.2 Failover Behavior
┌─────────────────────────────────────────────────────────────────────┐
│ Dynamic Border Failover │
├─────────────────────────────────────────────────────────────────────┤
│ │
│ Normal Operation: │
│ ┌─────────────────────────────────────────────────────────────┐ │
│ │ Edge-01 ───────────────────────────────────► Border-01 (Pri)│ │
│ │ Edge-02 ───────────────────────────────────► Border-01 (Pri)│ │
│ │ Edge-03 ───────────────────────────────────► Border-02 (Sec)│ │
│ │ Edge-04 ───────────────────────────────────► Border-02 (Sec)│ │
│ └─────────────────────────────────────────────────────────────┘ │
│ │
│ Border-01 Failure: │
│ ┌─────────────────────────────────────────────────────────────┐ │
│ │ Edge-01 ──────────────────────────────╳─► Border-01 (DOWN) │ │
│ │ └────────────────────────────────► Border-02 (Auto) │ │
│ │ Edge-02 ──────────────────────────────╳─► Border-01 (DOWN) │ │
│ │ └────────────────────────────────► Border-02 (Auto) │ │
│ │ Edge-03 ───────────────────────────────────► Border-02 │ │
│ │ Edge-04 ───────────────────────────────────► Border-02 │ │
│ └─────────────────────────────────────────────────────────────┘ │
│ │
│ Convergence Time: 2-5 seconds (with LISP Pub/Sub) │
│ │
└─────────────────────────────────────────────────────────────────────┘
6. SD-Access Transit Integration
6.1 Transit Border Requirements
When using SD-Access Transit for multi-site fabric:
Transit_Border_Requirements:
LISP_Pub_Sub:
- Required for transit functionality
- Enables cross-site route exchange
Dynamic_Default_Border:
- Required for optimal transit path selection
- Enables automatic failover between transit borders
Configuration:
- Both border nodes configured as Transit borders
- BGP peering to transit network (if applicable)
- VN extension across sites
6.2 Multi-Site Topology
┌─────────────────────────────────────────────────────────────────────┐
│ SD-Access Transit - Multi-Site │
├─────────────────────────────────────────────────────────────────────┤
│ │
│ MUMBAI SITE CHENNAI SITE │
│ ┌───────────────────┐ ┌───────────────────┐ │
│ │ MUM-BN-01 ◄──────────────────────► CHE-BN-01 │ │
│ │ (Transit Border) │ Transit │ (Transit Border) │ │
│ │ ▲ │ Network │ ▲ │ │
│ │ │ │ │ │ │ │
│ │ MUM-BN-02 ◄──────────────────────► CHE-BN-02 │ │
│ │ (Transit Border) │ │ (Transit Border) │ │
│ └───────────────────┘ └───────────────────┘ │
│ │
│ Dynamic Default Border ensures: │
│ • Traffic from Mumbai uses optimal border │
│ • Automatic failover if transit link fails │
│ • Load distribution across transit paths │
│ │
└─────────────────────────────────────────────────────────────────────┘
7. Verification Commands
7.1 Control Plane Verification
! On Control Plane Node
! Verify border registrations
show lisp instance-id * ethernet server
! Look for default route (0.0.0.0) registrations from borders
show lisp instance-id * ethernet publication
! Verify border nodes are publishing default route
show lisp instance-id * ethernet subscription
! Verify edge nodes are subscribed to default route
7.2 Edge Node Verification
! On Edge Node
! Verify default route received
show lisp instance-id 4097 ethernet map-cache
! Look for 0.0.0.0/0 entry pointing to border(s)
show ip route vrf VN_CORPORATE 0.0.0.0
! Verify default route in routing table
show ip cef vrf VN_CORPORATE 0.0.0.0/0
! Verify CEF entry for default route
7.3 Border Node Verification
! On Border Node
! Verify default route advertisement
show lisp instance-id * ethernet publication
! Verify this border is publishing default route
show ip route vrf VN_CORPORATE
! Verify external routes are present
show lisp session
! Verify LISP sessions to CP nodes
8. Troubleshooting
8.1 Common Issues
| Issue |
Cause |
Resolution |
| Edge not receiving default |
Border not marked as default |
Enable in Catalyst Center |
| Slow failover |
Legacy LISP (not Pub/Sub) |
Upgrade to Pub/Sub |
| Traffic asymmetry |
Priority mismatch |
Align border priorities |
| No load distribution |
Single border configured |
Add second border |
8.2 Debug Commands
! Enable LISP debugging (use sparingly)
debug lisp control-plane all
! Check LISP publication status
show lisp instance-id * ethernet publication detail
! Check LISP subscription status
show lisp instance-id * ethernet subscription detail
! Verify border to CP connectivity
show lisp session
Document Version: 2.0
Abhavtech.com - SD-Access Implementation