Came across this one while studying for the JNCIP-ENT.
Quick Summary:
- mtu-discovery is disabled by default
- TCP MSS for non-direct peerings: 512-byte
- thus convergence can be improved by enabling mtu-discovery
JNCIE-SP: IPv6 NLRIs over IPv4 BGP Peering When You're Not Using Mapped Addresses.
I previously posted about a way to configure BGP for IPv6 Unicast NLRI over an IPV4 session. Well, sometimes you don’t get to choose the address that the interface is running and it may be the ::1.2.3.4 ipv4-compatible style instead of the the ::ffff:mapped style.
This post is about how you get it to work.
In this scenario we have our PE, R1 and the CE, R3.
This is R1’s config.
set interfaces ge-0/0/2 unit 0 family inet address 172.27.0.5/30
set interfaces ge-0/0/2 unit 0 family inet6 address ::172.27.0.5/126
set protocols bgp group ebgp type external
set protocols bgp group ebgp family inet unicast
set protocols bgp group ebgp family inet6 unicast
set protocols bgp group ebgp peer-as 3
set protocols bgp group ebgp neighbor 172.27.0.6
This is R3’s config
set interfaces ge-0/0/2 unit 0 family inet address 172.27.0.5/30
set interfaces ge-0/0/2 unit 0 family inet6 address ::172.27.0.5/126
set protocols bgp group ebgp type external
set protocols bgp group ebgp family inet unicast
set protocols bgp group ebgp family inet6 unicast
set protocols bgp group ebgp export export-ebgp
set protocols bgp group ebgp peer-as 701
set protocols bgp group ebgp neighbor 172.27.0.5
set policy-options policy-statement export-ebgp term 1 from protocol aggregate
set policy-options policy-statement export-ebgp term 1 from rib inet6.0
set policy-options policy-statement export-ebgp term 1 from route-filter 3333:3333::/32 exact
set policy-options policy-statement export-ebgp term 1 then accept
This is the sad situation on R1 and R3:
lab@R1> show bgp summary
172.27.0.6 3 51 53 0 0 20:31 Establ
inet.0: 0/0/0/0
inet6.0: 0/0/0/0
lab@R1> show log messages | grep sanity
Feb 13 17:15:01 R1 rpd[1131]: bgp_nexthop_sanity: peer 172.27.0.6 (External AS 3) next hop ::ffff:172.27.0.6 unexpectedly remote, ignoring routes in this update
lab@R3> show log messages | grep sanity
Feb 13 21:46:27 R3 rpd[1135]: bgp_nexthop_sanity: peer 172.27.0.5 (External AS 701) next hop ::ffff:172.27.0.5 unexpectedly remote, ignoring routes in this update
Per standard, BGP sets the next-hop to it’s IPv4 address using ::ffff ipv4-mapped addressing. If we add “accept-remote-nexthop” R1’s BGP config, we get this:
lab@R1# activate protocols bgp group ebgp accept-remote-nexthop
[edit]
lab@R1# commit and-quit
commit complete
Exiting configuration mode
lab@R1> show bgp summary
172.27.0.6 3 58 61 0 0 23:06 Establ
inet.0: 0/0/0/0
inet6.0: 0/1/1/0
lab@R1> show route resolution unresolved
Tree Index 1
Tree Index 2
Tree Index 3
3333:3333::/32
Protocol Nexthop: ::ffff:172.27.0.6
Indirect nexthop: 0 -
We now see the route but it is not resolvable. So to fix it, we need to change the next hop to the inet6 address assigned to our peering interface. I’m going to fix both directions from R1’s policy since I am assuming no control over R3.
## the route we are advertising to R3 comes from IBGP, so we simply adjust the next-hop
set policy-options policy-statement export-ebgp term reset-v6-nexthop from protocol bgp
set policy-options policy-statement export-ebgp term reset-v6-nexthop from rib inet6.0
set policy-options policy-statement export-ebgp term reset-v6-nexthop then next-hop ::172.27.0.5
## we do similar handing for routes received from R3
set policy-options policy-statement import-ebgp from protocol bgp
set policy-options policy-statement import-ebgp from rib inet6.0
set policy-options policy-statement import-ebgp from next-hop ::ffff:172.27.0.6
set policy-options policy-statement import-ebgp then next-hop ::172.27.0.6
set policy-options policy-statement import-ebgp then accept
## apply policy configs to bgp
set protocols bgp group ebgp import import-ebgp
set protocols bgp group ebgp export export-ebgp
After we commit this config change on R1, we now have reachability both ways.
lab@R1> show bgp summary
Groups: 2 Peers: 2 Down peers: 0
Table Tot Paths Act Paths Suppressed History Damp State Pending
inet.0 0 0 0 0 0 0
inet6.0 2 2 0 0 0 0
Peer AS InPkt OutPkt OutQ Flaps Last Up/Dwn State|#Active/Received/Accepted/Damped...
10.255.1.32 701 94 97 0 0 41:04 Establ
inet.0: 0/0/0/0
inet6.0: 1/1/1/0
172.27.0.6 3 67 71 0 0 27:15 Establ
inet.0: 0/0/0/0
inet6.0: 1/1/1/0
lab@R1> show route table inet6 3333:3333::/32
inet6.0: 7 destinations, 8 routes (7 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
3333:3333::/32 *[BGP/170] 00:04:18, localpref 100, from 172.27.0.6
AS path: 3 I
> to ::172.27.0.6 via ge-0/0/2.0
And R3.
lab@R3> show bgp summary
Groups: 1 Peers: 1 Down peers: 0
Table Tot Paths Act Paths Suppressed History Damp State Pending
inet.0 0 0 0 0 0 0
inet6.0 1 1 0 0 0 0
Peer AS InPkt OutPkt OutQ Flaps Last Up/Dwn State|#Active/Received/Accepted/Damped...
172.27.0.5 701 77 72 0 0 29:23 Establ
inet.0: 0/0/0/0
inet6.0: 1/1/1/0
lab@R3> show route protocol bgp
inet.0: 13 destinations, 13 routes (13 active, 0 holddown, 0 hidden)
inet6.0: 8 destinations, 8 routes (8 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
4444:4444::/32 *[BGP/170] 00:02:55, localpref 100, from 172.27.0.5
AS path: 701 4 I
> to ::172.27.0.5 via ge-0/0/1.0
lab@R3> show route protocol bgp detail
inet.0: 13 destinations, 13 routes (13 active, 0 holddown, 0 hidden)
inet6.0: 8 destinations, 8 routes (8 active, 0 holddown, 0 hidden)
4444:4444::/32 (1 entry, 1 announced)
*BGP Preference: 170/-101
Next hop type: Router, Next hop index: 587
Address: 0x934c688
Next-hop reference count: 2
Source: 172.27.0.5
Next hop: ::172.27.0.5 via ge-0/0/1.0, selected
State:
Local AS: 3 Peer AS: 701
Age: 3:22
Task: BGP_701.172.27.0.5+52965
Announcement bits (1): 0-KRT
AS path: 701 4 I Aggregator: 4 10.255.1.34
Accepted
Localpref: 100
Router ID: 10.255.1.31
There you have it: accept-remote-nexthop, and some resetting of the next-hop works by either import or export policy.