Checking your BGP MSS on Junos and IOS

Oddly enough, neither platform shows you directly what your MSS between your BGP speakers is. Quite annoying. To quickly check on Junos do this:

[email protected]> show system connections extensive |  match "179|mss"
tcp4       0      0  10.250.8.9.59064                         10.0.1.89.179
    rttmin:       1000  mss:        512
tcp4       0      0  10.0.0.1.65081                         10.1.0.1.179
    rttmin:       1000  mss:        492
tcp4       0      0  10.10.0.1.56298                            10.20.0.1.179
    rttmin:       1000  mss:       1440

For IOS its quite similar:

par8.the1#show tcp      | include .179|Datagrams
Foreign host: 10.20.30.1, Foreign port: 179
Datagrams (max data segment is 556 bytes):
Foreign host: 10.30.20.1, Foreign port: 179
Datagrams (max data segment is 8860 bytes):

Would be nice of this was show in the actual BGP neighbourship output, but BGP does ride on top of TCP so…

EDIT: Actually IOS does show this in a neighbour command:

R2#show bgp vpnv4 unicast all neighbors 19.19.19.19 | include Data
Datagrams (max data segment is 1460 bytes):

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