Introduction
ASR9K XR and ASR903 (IOSXE) ISIS behaviour
Problem
ASR9K preferring ISIS L2 for prefix (Lo0) of peer ASR903 (IOS-XE) due to which RAN instance interface is getting preferred
Solution
IOS XR & IOSXE behaviour
IOSXE & IOSXE behaviour
Explanation
- Comparison of L1/L2 routes within ISIS is same in XE/XR, it’s defined by RFC1195. Level 1 is preferred
- Comparison of routes between two ISIS processes (done by RIB) is not standardized and there may be difference in XE and XR given the nature of both OS-es
- Because the Routing Information Base (RIB) treats each of the IS-IS instances as equal routing clients, you must be careful when redistributing routes between IS-IS instances. The RIB does not know to prefer Level 1 routes over Level 2 routes. For this reason, if you are running Level 1 and Level 2 instances, you must enforce the preference by configuring different administrative distances for the two instances.
- However, in both OS-es it’s good practice to set admin-distance for preferring ISIS instance.
So, from RIB POV
- If a prefix have same metric learned on 2 different process , it uses tradition method of Administrative distance to break the tie.
- If a prefix is learned on 2 different process & AD is same, it uses metric to break the tie.
XR documentation
To enforce the preference by configuring different administrative distances for the two instances in ASR9K.
ASR9K-XR Configuration
router isis CORE
set-overload-bit on-startup 360
is-type level-1-only
net <net-id>
address-family ipv4 unicast
metric-style wide
attached-bit send never-set
distance 114 10.1.1.1/32 test-XR >>> To prefer CORE instance for the Lo0 prefix learnt from neighbour XE/XR node
spf prefix-priority high tag 10
prefix-set test-XR
10.1.1.1/32
end
router isis RAN
set-overload-bit on-startup 360
is-type level-2-only
net <net-id>
address-family ipv4 unicast
metric-style wide
attached-bit send never-set
redistribute isis CORE level-2 route-policy TEST-IPv4
maximum-redistributed-prefixes 10000 level 2
spf prefix-priority high tag 10