OSPF#
OSPF Packet Types#
| Type |
Name |
Purpose |
| 1 |
Hello |
Neighbor discovery, DR/BDR election, keepalive |
| 2 |
DBD |
LSDB table of contents exchange |
| 3 |
LSR |
Request specific missing LSAs |
| 4 |
LSU |
Delivers actual LSAs |
| 5 |
LSAck |
Confirms LSU receipt |
LSA Types#
| Type |
Name |
Originated By |
Scope |
Blocked By |
| 1 |
Router |
Every router |
Area |
— |
| 2 |
Network |
DR |
Area |
— |
| 3 |
Summary |
ABR |
Domain |
Totally Stubby, Totally NSSA |
| 4 |
ASBR Summary |
ABR |
Domain |
Totally Stubby, Totally NSSA, NSSA, Stub |
| 5 |
AS-External |
ASBR |
Domain |
All stub and NSSA areas |
| 7 |
NSSA External |
ASBR (NSSA) |
NSSA only |
— (translated to Type 5 at ABR) |
Area Types#
| Area |
Type 3 |
Type 4 |
Type 5 |
Type 7 |
Default Route |
| Standard |
Yes |
Yes |
Yes |
No |
No |
| Backbone |
Yes |
Yes |
Yes |
No |
No |
| Stub |
Yes |
No |
No |
No |
Yes (auto) |
| Totally Stubby |
No |
No |
No |
No |
Yes (auto) |
| NSSA |
Yes |
No |
No |
Yes |
No (configurable) |
| Totally NSSA |
No |
No |
No |
Yes |
Yes (auto) |
Adjacency States#
Down → Init → 2-Way → ExStart → Exchange → Loading → Full
IS-IS#
PDU Types#
| PDU Type |
Name |
Sent By |
Network Type |
Purpose |
| IIH |
IS-IS Hello |
All Routers |
LAN or P2P |
Establishes and maintains adjacencies. P2P hellos cover both levels; LAN hellos are level-specific. |
| LSP |
Link State PDU |
All Routers |
All |
The IS-IS equivalent of an LSA. Carries routing info (prefixes, neighbors, metrics) in TLVs. Flooded per level. |
| CSNP |
Complete Sequence Number PDU |
DIS (on LAN) |
LAN or P2P |
Acts as a table of contents for the LSDB. Sent periodically on LANs (default 10s) to keep routers in sync. |
| PSNP |
Partial Sequence Number PDU |
Any Router |
LAN or P2P |
Requests a missing LSP or acknowledges receipt of an LSP (on P2P links only). |
TLV Types#
| TLV Type |
Name |
Primary Usage |
| 1 |
Area Addresses |
Adjacency formation |
| 22 |
Extended IS Reachability |
Wide metrics for neighbors |
| 129 |
Protocols Supported |
IPv4 / IPv6 capability |
| 135 |
Extended IP Reachability |
Wide metrics for IPv4 prefixes |
| 236 |
IPv6 Reachability |
IPv6 prefix advertisements |
| 242 |
Router CAPABILITY |
Advertising SR/TE features |
MPLS#
Label Operations#
| Operation |
Who Does It |
Description |
| Push |
Ingress LSR |
Adds a label to the packet |
| Swap |
Transit LSR |
Replaces the top label |
| Pop |
Egress or penultimate hop |
Removes the top label |
Reserved Labels#
| Label |
Name |
Meaning |
| 0 |
IPv4 Explicit Null |
Signal QoS intent to egress |
| 1 |
Router Alert |
Trap to local CPU |
| 2 |
IPv6 Explicit Null |
Signal QoS intent for IPv6 |
| 3 |
Implicit Null |
Trigger PHP at penultimate hop |
LDP vs RSVP#
| Feature |
LDP |
RSVP |
| Traffic Engineering |
No |
Yes |
| Explicit paths |
No |
Yes (ERO) |
| Bandwidth reservation |
No |
Yes |
| Path calculation |
IGP topology |
CSPF |
| LSPs created |
All prefixes (loopbacks by default) |
Manually configured |
| Soft-state |
No |
Yes (requires refresh) |
| Fast Reroute |
No |
Yes |
| Use case |
iBGP next-hop resolution |
TE, bandwidth guarantees |
Key Junos Routing Tables#
| Table |
Contents |
inet.0 |
Standard IP routes |
inet.3 |
MPLS-signaled routes (BGP next-hop resolution) |
mpls.0 |
Label forwarding entries (LFIB) |
Monitoring Commands#
| Command |
Purpose |
show mpls lsp |
Summary of all LSPs |
show mpls lsp ingress extensive |
Detailed ingress LSP info including RRO |
show rsvp session |
Active RSVP sessions |
show rsvp interface |
RSVP interface state and bandwidth |
show ldp neighbor |
LDP neighbor adjacencies |
show ldp session |
LDP TCP sessions |
show ldp database |
LDP label bindings |
show ldp interface |
LDP-enabled interfaces |
show route table inet.3 |
MPLS routes used for BGP resolution |
show route table mpls.0 |
MPLS forwarding table |
traceroute mpls ldp <prefix> |
Trace an LDP LSP path |
Tunnels#
IP-IP vs GRE#
| Feature |
IP-IP |
GRE |
| Overhead |
20 bytes |
24 bytes (minimum) |
| IP protocol number |
4 |
47 |
| IPv4 unicast |
Yes |
Yes |
| IPv6 |
No (use 6in4 proto 41) |
Yes |
| Multicast |
No |
Yes |
| MPLS |
No |
Yes |
| L2 frames |
No |
Yes |
| Keepalives |
No |
Yes |
| Junos interface |
ip-0/0/0 |
gr-0/0/0 |
MTU Impact#
| Tunnel |
Overhead |
Effective MTU (1500B link) |
| IP-IP |
20B |
1480 |
| GRE |
24B |
1476 |
| GRE + 1 MPLS label |
28B |
1472 |
Key Commands#
| Command |
Purpose |
show interfaces gr-0/0/0 detail |
Tunnel state, counters |
show interfaces gr-0/0/0 extensive |
Includes keepalive stats |
ping <dest> size 1472 do-not-fragment |
MTU path test |
High Availability#
HA Technology Comparison#
| Feature |
GR |
GRES |
NSR |
ISSU |
| Protects against |
Control plane restart |
RE hardware failure |
RE hardware failure |
Software upgrade |
| Requires dual RE |
No |
Yes |
Yes |
Yes |
| Requires GRES |
No |
— |
Yes |
Yes |
| Requires NSR |
No |
No |
— |
Yes |
| Neighbor cooperation needed |
Yes |
No |
No |
No |
| Forwarding interrupted |
No |
No |
No |
No |
| Protocol reconvergence |
No (helpers hold routes) |
Yes |
No |
No |
| Transparent to neighbors |
No |
No |
Yes |
Yes |
LACP Modes#
| Mode |
Behavior |
| Active |
Initiates LACP — sends PDUs unconditionally |
| Passive |
Responds only — at least one side must be active |
BFD Detection Time#
Detection time = negotiated-interval × multiplier
Negotiated interval = max(local min-interval, remote min-interval)
Key Commands#
| Command |
Purpose |
show lacp status |
LAG member link states |
show interfaces ae0 detail |
AE interface stats and member links |
show system switchover |
GRES status |
show task replication |
NSR sync status |
show chassis routing-engine |
RE status and mastership |
show bfd session |
BFD session states and timers |
Protocol Independent Routing#
Default Route Preferences#
| Protocol |
Preference |
| Direct / Local |
0 |
| Static |
5 |
| OSPF Internal |
10 |
| IS-IS L1 Internal |
15 |
| IS-IS L2 Internal |
18 |
| RIP |
100 |
| Aggregate / Generated |
130 |
| OSPF External |
150 |
| IS-IS L1 External |
160 |
| IS-IS L2 External |
165 |
| BGP |
170 |
Static Route Next-Hop Options#
| Next-hop |
Behavior |
| IP address |
Forward to next-hop |
reject |
Drop + ICMP unreachable |
discard |
Silent drop |
next-table |
Redirect to another routing table |
Routing Instance Types#
| Type |
Purpose |
forwarding |
FBF — separate forwarding table, no protocols |
virtual-router |
Full isolated routing domain with protocols |
vrf |
MPLS L3VPN |
vpls |
MPLS L2VPN (multipoint) |
l2vpn |
MPLS L2VPN (point-to-point) |
Key Commands#
| Command |
Purpose |
show route |
Show active routing table |
show route hidden |
Show inactive / suppressed routes |
show route martians table inet.0 |
Show martian prefix list |
show route instance |
List all routing instances |
show route <prefix> exact detail |
Show a specific route with contributing details |
show route forwarding-table |
Show the forwarding table (LFIB) |
Layer 2 / VLANs#
802.1ad Tag Operations#
| Operation |
Where |
Description |
| Push |
Ingress PEB |
Add S-Tag to customer frame |
| Pop |
Egress PEB |
Remove S-Tag |
| Swap |
Inter-provider handoff |
Replace S-Tag value |
STP Port States#
| State |
BPDUs |
Learn MACs |
Forward Data |
| Blocking |
Rx only |
No |
No |
| Listening |
Yes |
No |
No |
| Learning |
Yes |
Yes |
No |
| Forwarding |
Yes |
Yes |
Yes |
RSTP Port Roles#
| Role |
Description |
| Root |
Best path to root |
| Designated |
Best port on segment toward root |
| Alternate |
Backup root port — instant failover |
| Backup |
Backup designated port (same bridge, same segment) |
STP Timers#
| Timer |
Default |
Impact |
| Hello |
2s |
BPDU send interval |
| Forward Delay |
15s |
Time in Listening + Learning states |
| Max Age |
20s |
BPDU expiry |
Key Commands#
| Command |
Purpose |
show bridge mac-table |
Layer 2 forwarding table |
show bridge domain |
Bridge domain summary |
show spanning-tree bridge detail |
STP topology and root info |
show spanning-tree interface |
Per-port STP state and role |
IPv6#
Address Types#
| Prefix |
Type |
Routable |
2000::/3 |
Global Unicast |
Yes |
FC00::/7 |
Unique Local |
No (private) |
FE80::/10 |
Link-Local |
No (link only) |
FF00::/8 |
Multicast |
Scope-dependent |
::1/128 |
Loopback |
No |
NDP Message Types#
| Message |
ICMPv6 |
Purpose |
| RS |
133 |
Host solicits router |
| RA |
134 |
Router announces prefix/config |
| NS |
135 |
MAC resolution / DAD |
| NA |
136 |
Response to NS |
| Redirect |
137 |
Better next-hop notification |
Address Assignment Methods#
| Method |
RA M flag |
RA O flag |
Address Source |
| SLAAC |
0 |
0 |
Self-generated |
| SLAAC + DHCPv6 |
0 |
1 |
Self-generated + DHCPv6 options |
| DHCPv6 stateful |
1 |
— |
DHCPv6 server |
OSPFv2 vs OSPFv3#
| Feature |
OSPFv2 |
OSPFv3 |
| Junos hierarchy |
protocols ospf |
protocols ospf3 |
| Addressing in Router LSA |
Yes |
No (Type 8/9) |
| Router ID required |
Auto or manual |
Manual if no IPv4 |
| Runs over |
IPv4 |
IPv6 link-local |
OSPFv3 LSA Types for IPv6#
| Type |
Name |
Purpose |
| 8 |
Link LSA |
Link-local address + prefixes on the link |
| 9 |
Intra-Area Prefix LSA |
IPv6 prefix reachability within an area |